1. Field of the Invention
The present invention relates to a cathode-ray tube (CRT) video drive circuit, and more specifically, to an improved video amplifier circuit that combines high bandwidth with beam current stabilization.
2. Description of the Prior Art
Cathode-ray tubes (CRTs) have been used extensively in various electronic products, such as television receivers, oscilloscopes, and other display units because of their high brightness, quick response, and high resolution, among many other inherent advantages. In raster scan display devices, such as television receivers, the control grid (G.sub.1) is driven by the output signal from a video amplifier. The output signal will control the cathode (K)-to-grid G.sub.1 voltage thereby controlling the beam current over a certain range, the beam current modulating the phosphor luminescence while the electron beam is scanning across the phosphor screen.
The cutoff bias voltage of a CRT is affected by the heater temperature of the cathode, the cathode-to-grid G.sub.1 spacing, cathode aging, and the second grid (G.sub.2) voltage. Any change in the cutoff bias voltage will result in a different brightness of the displayed image on the CRT, and it will affect the low light color balance of the display if a color CRT is used.
Some circuits regulate the control grid-to-cathode voltage within close tolerances. However, warm-up and aging characteristics of the cathode still tend to modify the grid-to-cathode cutoff voltage, thereby affecting the beam current and, accordingly, the brightness of the displayed image.
In some prior art video amplifiers, the cathode current is monitored. A feedback loop connects the monitored cathode current to the input of an amplifier. The amplifier is connected to the control grid G.sub.1 of the CRT. When the cathode current varies from a desired level, the input to the amplifier is changed in such a way as to modify the control grid voltage in order that the brightness of the displayed image, or the color balance of the display will be maintained at its desired level. However, with such a circuit configuration, the amplifier has a limited bandwidth because it is necessary to maintain the dynamic range of the output signal at a certain level.